How does rising atmospheric CO2 affect marine organisms?

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Marine Phytoplankton and Clouds
Meskhidze, N. and Nenes, A. 2006. Phytoplankton and cloudiness in the Southern Ocean. Science 314: 1419-1423.

The authors introduce their impressive study by noting that "marine aerosols strongly affect properties and lifetime of stratiform clouds, influencing earth's radiation budget and climate," and by adding that the "production of sulfate from the oxidation of dimethylsulfide (DMS), proposed by Shaw (1983) and explored by Charlson et al. (1987), and primary emissions of biogenic organic matter from wave breaking (Middlebrook et al., 1998; O'Dowd et al., 2004) have been suggested as possible mechanisms by which phytoplankton can modulate properties of marine clouds."

What was done
Over a continuous six-year period, Meskhidze and Nenes explored the effects of ocean biological productivity on the microphysical and radiative properties of marine clouds over a large and seasonally-recurring phytoplankton bloom in the Southern Ocean in the vicinity of South Georgia Island, where upwelling nutrient-rich waters "can support massive phytoplankton blooms, with chlorophyll a concentrations more than an order of magnitude higher than the background." In this endeavor, they used the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) to obtain the needed chlorophyll data and the Moderate Resolution Imaging Spectroradiometer (MODIS) to determine the effective radii of cloud condensation nuclei.

What was learned
The researchers' efforts revealed that "cloud droplet number concentration over the bloom was twice what it was away from the bloom, and cloud effective radius was reduced by 30%," such that "the resulting change in the short-wave radiative flux at the top of the atmosphere was -15 watts per square meter, comparable to the aerosol indirect effect over highly polluted regions," and, we might add, much greater locally than the opposite (positive) radiative forcing calculated to have been produced by the increasing concentrations of all greenhouse gases emitted to the atmosphere since pre-industrial times.

What it means
In what amounts to a massive understatement of the major implication of their findings, Meskhidze and Nenes conclude that secondary organic aerosol formation in remote marine air may need to be included in global climate models, as it may play, as they describe it, "a considerable role in climate transition[s]," which role just happens to be one of powerful negative feedback.

Charlson, R.J., Lovelock, J.E., Andrea, M.O. and Warren, S.G. 1987. Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326: 655-661.

Middlebrook, A.M., Murphy, D.M. and Thomson, D.S. 1998. Observations of organic material in individual marine particles at Cape Grim during the First Aerosol Characterization Experiment (ACE 1). Journal of Geophysical Research 103: 16,475-16,484.

O'Dowd, C.D., Facchini, M.C., Cavalli, F., Ceburnis, D., Mircea, M., Decesari, S., Fuzzi, S., Yoon, Y.J. and Putaud, J.-P. 2004. Biogenically driven organic contribution to marine aerosol. Nature 431: 676-680.

Shaw, G.E. 1983. Bio-controlled thermostasis involving the sulfur cycle. Climatic Change 5: 297-303.

Reviewed 14 March 2007